The present invention relates to passive radio frequency identification (RFID) tags, and in particular, to improved power efficiencies in passive RFID tags by minimizing excess power consumption.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Typical RFID tags consist of an RFID chip mounted on an inlay with an antenna. The antenna is tuned to maximize sensitivity (and hence read range) at a specified operating frequency. In normal operation, an excitation source (e.g., RFID reader) generates a carrier frequency that energizes the RFID tag and bi-directional data is superposed on this carrier. One or two volts is usually sufficient to power the RFID chip.
When a tag is very close to the excitation source, the voltage generated by its antenna can potentially exceed the rating of the RFID circuit. A shunt regulator is the most common way to protect the circuit against this potential source of damage. This shunt regulator protects the RFID circuit by dumping the excess energy as heat. For most RFID applications, this energy loss does not meaningfully detract from system performance.
Some RFID applications, however, require reading tags that are closely spaced. Unfortunately, closely spaced RFID tags tend to couple, shifting their resonance frequency from that of the carrier to some unknown frequency. This shift causes a breakdown in the transfer of energy and data between reader and tags. The more closely coupled the tags, the greater the shift in resonance, and the greater the degradation in system performance.
One solution that has been developed—with limited results—is to not tune the RFID tag to minimize the coupling between adjacent tags. See, for example, products from Magellan Technology Pty Ltd, Sydney, Australia. However, this solution has poor spatial discrimination. Attempts to improve spatial discrimination may themselves introduce further issues, such as collapsing the field and severely limiting the read range. To counter the limited read range, various range extension solutions may be implemented. Unfortunately, when these range extension solutions are combined with closely spaced “off-the-shelf” RFID tags, the shunt regulator exhibits a self-limiting behavior. Specifically, the shunt regulator in the tags nearest the excitation source (the reader) clamp the voltage at the output of the coil antenna in the tag. Since the tags are closely spaced, the voltage available to nearby tags is limited to this clamped voltage. Any losses (and there are always losses) further reduce the available voltage to where the tags do not have sufficient power to operate. As noted above, this shunt regulator acts to protect the RFID circuit (a good thing) but—in the case of closely spaced tags—the shunt regulator reduces the read range of the system.
Thus, there is a need for circuits that protect the sensitive circuitry in RFID tags without burning up excess energy as heat.